1. Field of the Invention
The present invention relates to a braking force control system of a vehicle which adequately distributes the braking force in braking.
2. Description of the Related Art
In recent years, there have been proposed various braking control systems which make use of constitutions of anti-lock braking systems (ABS) and control distribution of braking force in normal braking to be more adequate one.
For example, in JP-A-10-119744, there is disclosed one which, when deceleration of a vehicle reaches a specified value and a vehicle speed exceeds a specified speed, independently increases, reduces or holds braking oil pressure of each of right and left rear wheels so that braking force distribution between rear and front wheels becomes an approximately ideal one to perform independent braking control for carrying out sufficient braking.
During a braking operation, for example, road surface friction coefficients (road surface friction) with respect to left and right wheels are greatly asymmetric in some cases. Namely, when a wheel on one side of a vehicle is on a dry road surface (high friction road) and a wheel on the other side thereof is on, for example, an ice plate (low friction road), a very high yawing moment occurs in some cases when the vehicle is braked.
Therefore, in a related art ABS control operation, in order to prevent the stability of the vehicle from being lost by applying a large braking force to a wheel on the side of a high friction road, a select-low control, in which a brake on a high friction road side is controlled in accordance with a brake on a low friction road side, i.e., on the side of a large slipping condition, is employed in many cases.
In general, the above-described independent braking control is suitable for a case where the obtainment of a sufficient braking force is required, while the above-described select-low control is suitable for a case where the minimization of the occurrence of a yawing moment and the obtainment of a sufficient vehicle stability are required. Therefore, it is desired that these controls can be carried out in a suitably selected manner in accordance with the variously changing traveling condition of a vehicle.
Further, in a braking operation, also important is a method of generating a braking pressure for producing a braking force. Especially, when a braking pressure is rapidly increased on a split friction road or with a load on a vehicle greatly different at left and right parts thereof, and in other similar cases, a yawing moment occurs in the vehicle. This would cause the vehicle to become unstable, or an unpleasant rolling behavior to be left during the turning with a high lateral acceleration.
The present invention has been made in view of the above-mentioned circumstances, and provides a braking force distribution control apparatus for vehicles, capable of carrying out a reliable and stable braking operation finely and in a well-balanced manner by suitably selecting various braking controls in accordance with variously changing traveling condition of a vehicle, and executing a selected control, without adding a special complicated mechanism to the apparatus.
To attain the above object, according to a first aspect of the invention, there is provided a braking force distribution control apparatus for a vehicle, including: a lateral acceleration detecting unit detecting a lateral acceleration of the vehicle; a longitudinal acceleration detecting unit detecting a longitudinal acceleration of the vehicle; a vehicle speed detecting unit detecting a vehicle speed; and a braking control unit adapted to select and execute, one of a select-low control and an independent braking control in accordance with the lateral acceleration, the longitudinal acceleration and the vehicle speed when preset conditions for brake operating time are satisfied, the select-low control controlling braking forces of left and right wheels depending on a wheel on the side with a large slipping condition, the independent braking control independently controlling the braking force for each wheel depending on the slipping condition of each wheel.
In the braking force distribution control apparatus according to the first aspect, the lateral acceleration of a vehicle is detected by the lateral acceleration detecting unit, the longitudinal acceleration of the vehicle by the longitudinal acceleration detecting unit, and a vehicle speed by the vehicle speed detecting unit. The braking control device is adapted to select, when preset conditions for a braking operation are satisfied, either of a select-low control in which a wheel braking force for a left wheel and a right wheel is controlled in accordance with lateral acceleration, longitudinal acceleration and vehicle speed and correspondingly to a wheel on the side with a large slipping condition, and an independent braking control in which the braking force for each wheel is controlled independently and correspondingly to the slipping condition of each wheel, and then execute a selected control.
In addition, in the above braking force distribution control apparatus, when the preset conditions for a braking operation are satisfied, the braking control unit is adapted to select, either of the select-low control and the independent braking control in accordance with the vehicle speed in a case where the lateral acceleration is in a preset high region and then execute the selected control; select either of the select-low control and the independent braking control in accordance with the vehicle speed and the longitudinal acceleration in a case where the lateral acceleration is in a preset intermediate region, and then execute the selected control; and select and execute the select-low control in a case where the lateral acceleration is in a preset low region.
This braking force distribution control apparatus is so constituted that, for example, when the lateral acceleration is in a preset high region with a vehicle speed being low, a sufficient braking operation can be carried out by selectively executing an independent braking control, and so that, when the lateral acceleration is in the same region with a vehicle speed being high, the safety can be secured by selectively executing a select-low control. The apparatus is also so constituted that, when the lateral acceleration is in a preset intermediate region with the longitudinal acceleration being high, a judgement that a vehicle is traveling on a high friction road is given; when a vehicle speed is low, a sufficient braking operation can be carried out by executing an independent braking control; and, when a vehicle speed is high, the stability can be secured by selectively executing a select-low control. The apparatus is also so constituted that, when the longitudinal acceleration is low in this case, the stability can be secured by selectively executing a select-low control. The apparatus is formed so that, when the lateral acceleration is in a preset low region, the stability can be secured by selectively executing a select-low control.
Further, in the above braking force distribution control apparatus according to the first aspect, the select-low control and the independent braking control are carried out for the rear wheels.
Moreover, according to a second aspect of the invention, there is provided a braking force distribution control apparatus for a vehicle, including: a lateral acceleration detecting unit detecting a lateral acceleration of the vehicle; a longitudinal acceleration detecting unit detecting a longitudinal acceleration of the vehicle; and a braking control unit adapted to select and execute at least one of a select-low control, an independent braking control and a stepwise pressure increase control in accordance with the lateral acceleration and the longitudinal acceleration when preset conditions for brake operating time are satisfied, the select-low control controlling braking forces of left and right wheels depending on a wheel on the side with a large slipping condition, the independent braking control independently controlling the braking force for each wheel depending on the slipping condition of each wheel, and the stepwise pressure increase control stepwisely increasing a braking pressure to a predetermined value at the braking operation starting time.
Namely, in this braking force distribution control apparatus, the lateral acceleration of a vehicle is detected by the lateral acceleration detecting device, and the longitudinal acceleration thereof by the longitudinal acceleration detecting device. The braking control device is adapted to select one of, or to combine together such control operations in accordance with the lateral acceleration and longitudinal acceleration, when preset conditions for the brake operating time are satisfied, that include a select-low control in which a braking force for left and right wheels is controlled in accordance with a wheel on the side of a worse slipping condition, an independent braking control in which a braking force for each wheel is controlled independently in accordance with the slipping condition of each of these wheels, and a stepwise pressure increase control in which a braking pressure is stepwise increased to a value to be reached at the braking operation starting time, and execute a selected or combined control.
In addition, in the above braking force distribution control apparatus, the braking control device judges as to whether the detected lateral acceleration is in a high lateral acceleration region not lower than a predetermined value or in a low lateral acceleration region lower than the predetermined value, when the detected lateral acceleration is in the low lateral acceleration region, at least one of the select-low control, the independent braking control and the stepwise pressure increase control are selected in accordance with the lateral acceleration and the longitudinal acceleration, the selected control is then executed, and when the lateral acceleration is in the high lateral acceleration region, one of the independent braking control and a combination of the independent braking control and the stepwise pressure increase control is executed in accordance with the lateral acceleration.
When the lateral acceleration is in a low lateral acceleration region in this braking force distribution control apparatus for vehicles, the turning condition of a vehicle, the braking condition thereof and a road surface friction xcexc on which the vehicle travels are estimated on the basis of the lateral acceleration and longitudinal acceleration. For example, when the lateral acceleration is low with the longitudinal acceleration high, a result that the braking of the vehicle is done on a high friction road is drawn by inference, and both the front and rear wheels are subjected to an independent braking control, a reliable braking operation being thereby rendered practicable with a sufficient braking force. For example, in a case where the lateral acceleration is high with the longitudinal acceleration also high, the rear wheels and front wheels are subjected, when the possibility that the vehicle is traveling on a split friction road in an intermediate friction road can be assumed, to a select-low control and an independent control with which a stepwise pressure increase control is combined, respectively, the safety of the vehicle being thereby satisfactorily secured. For example, in a case where the lateral acceleration is low with the longitudinal acceleration also low, the rear wheels and front wheels are subjected, when the possibility that the vehicle is traveling on a split friction road in an intermediate friction road can be assumed, to a select-low control and an independent control with which a stepwise pressure increase control is combined, respectively, the safety of the vehicle being thereby satisfactorily secured. For example, when the lateral acceleration is high with the longitudinal acceleration low, a conclusion that the vehicle is turned with a gentle braking force on a high xcexc road or turned on an intermediate friction road is drawn by inference, and both the front and rear wheels are subjected to an independent braking control, a reliable braking operation being thereby rendered practicable with a sufficient braking force. In a case where the lateral acceleration is in a high lateral acceleration region, the turning condition of the vehicle is assumed in accordance with the lateral acceleration. For example, when the lateral acceleration is low with a conclusion that the vehicle is in a normal turning condition possible to be drawn by inference, both the front and rear wheels are subjected to an independent braking control, and a reliable braking operation can thereby be carried out with a sufficient braking force. Furthermore, when a quick turning is presumed with high lateral acceleration, the front wheels are subjected to an independent braking control while the rear wheels are subjected to an independent braking control combined with a stepwise pressure increase control to thereby suppress uncomfortable rolling impression.
Further, in the above braking force distribution control apparatus, the braking force control unit is adapted to execute the independent braking control for the front wheels, and further combine the independent braking control for the front wheels with the stepwise pressure increase control in accordance with the lateral acceleration and longitudinal acceleration. Namely, an independent braking control is carried out for the front wheels, and the reliable braking of the vehicle is thereby carried out with a sufficient braking force, while, in order to brake the vehicle with the stability thereof maintained by a combination of the lateral acceleration and longitudinal acceleration, a control of a combination of the independent braking control and a stepwise pressure increase control is carried out.
Moreover, in the above braking force distribution control apparatus, the braking control device is adapted to execute one of the select-low control and the independent braking control for the rear wheels in accordance with the lateral acceleration and longitudinal acceleration; and further combine the independent braking control for the rear wheels with the stepwise pressure increase control in accordance with the lateral acceleration and longitudinal acceleration. Namely, in order to brake the rear wheels with, for example, the stability of the vehicle maintained, a select-low control is selected, and, in order to brake the vehicle with the occurrence of unpleasant rolling behavior thereof held down, a stepwise pressure increase control is combined with the independent braking control.